The present disclosure generally relates to plasma generators employed in semiconductor manufacturing processes, and more particularly, to hydrocarbon dielectric heat transfer fluids, i.e., coolants, for use in microwave plasma generators.
Dielectric heat transfer fluids, i.e., coolants, can be used in many applications. One such application is for cooling dielectric barriers that are in direct contact with plasma. Plasma generators are frequently employed in semiconductor manufacture and it is generally required to employ dielectric heat transfer fluids to minimize the thermal stresses caused by the plasma on various components of the equipment.
A subset of plasma generators are microwave plasma generators. A microwave plasma asher typically includes a plasma tube in communication with a microwave source and a process chamber. The length of the tube is selected to encourage recombination of the more energetic particles along the length of the tube, forming stable, less damaging atoms and compounds. For example, less reactive F and O radicals reach the process chamber downstream of the microwave plasma source in greater proportions than high-energy ions. Because the process chamber is located downstream of the plasma source, this arrangement is generally referred to as a downstream plasma reactor. By creating a bend in the tube, the process chamber can be kept out of direct line of sight with the plasma, such that harmful UV radiation from the glow discharge does not reach the substrate.
The tube itself, however, places several limitations on the reactor. Conventionally, both the applicator and the transport tube are formed of quartz. Quartz exhibits advantageously low rates of O and F recombination, permitting these desired radicals to reach the process chamber while ions generated in the plasma source recombine. Unfortunately, quartz is highly susceptible to fluorine attack. Thus, the quartz transport tube and particularly the quartz applicator, which is subject to direct contact with the plasma, deteriorates rapidly and must be frequently replaced. Each replacement of the quartz tubing not only incurs the cost of the tubing itself, but more importantly leads to reactor downtime during tube replacement, and consequent reduction in substrate throughput.
An alternative material for applicators and/or transport tubes is sapphire (Al2O3). While highly resistant to fluorine attack, sapphire tubes have their own shortcomings. For example, sapphire transport tubes exhibit much higher rates of O and F recombination as compared to quartz, resulting in lower ash rates. Additionally, sapphire is susceptible to cracking due to thermal stresses created by the energetic plasma, limiting the power that can be safely employed. Lower plasma power means less generation of free radicals, which in turn also reduces the ash rate. While employing single-crystal sapphire somewhat improves the chemical resistance to fluorine relative to polycrystalline sapphire, safe power levels for single-crystal sapphire are still low compared to those which can be employed for quartz tubes. Moreover, bonding material at the joint between sapphire sections that create the bend in the transport tube, which prevents UV radiation from reaching the process chamber, is typically as susceptible to fluorine ion attack as is quartz.
In order to effectively minimize the thermal stresses, the plasma tube can be cooled with a dielectric heat transfer fluid or air. During operation, the dielectric heat transfer fluid or air is fed between two concentric tubes to provide cooling of the plasma tube. The primary purpose of the dielectric heat transfer fluid is to reduce thermal stresses within the plasma tube so as to prevent cracking. These thermal stresses are created by temperature differences across the tube geometry, which are generally caused by rapid and frequent collisions between energized particles and between such particles and the plasma tube walls.
Numerous properties are desired for dielectric heat transfer fluids to provide cooling to a microwave plasma device such as, for example, microwave transparency, which is generally indicated by a loss tangent function or dissipation factor. Absorption of microwave energy by the heat transfer fluid results in inefficiencies that can increase process times as well result in poor heat transfer since the absorption of microwave energy can cause heating of the fluid. Moreover, microwave absorption can also cause fragmentation of the fluid rendering the fluid unacceptable for its intended purpose. The fragments can cause an increase in dielectric constant as well as deleteriously affect the various fluid properties.
Apart from the use of air as a cooling medium, dielectric heat transfer fluids that have been or are currently employed for microwave plasma devices are perfluorinated liquids. While perfluorinated liquids are generally considered adequate for the intended use, these fluids are relatively expensive. Moreover, those perfluorinated liquids that are considered suitable in terms of its thermal, physical, chemical, and safety properties, generally have high vapor pressures. As such, perfluorinated fluids have a propensity to leak from closed loop systems. Although from a safety standpoint the loss of fluid may be acceptable, the costs of these fluids are relatively expensive. In addition, many of these perfluorinated fluids have relatively low boiling points making them susceptible to thermal breakdown, which can lead to reactive fragments. The reactive fragments can present health hazards as well as degrade the materials employed along the fluid path, e.g., o-rings, aluminum, leak lock thread sealants, polyurethane tubing, and the like. Consequently, these fluids can cause long-term performance issues and generally require frequent maintenance and inspection.
Accordingly, a need exists for alternative coolants that are microwave transparent, have adequate heat transfer capabilities, are environmentally friendly, and can provide lower costs of ownership.